Anodic oxidation of ferrophosphorus



Patented Sept.19, 1939 UNITED STATES- PATENT oral-cs snomo oxma'rron or manornosrnoans Lewis B. 1). Fraser, Sheflield,

No Drawing.

App cation October 15, 1938, Serial N0. 1053]..

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This invention relates to a process for the anodic oxidation of retro-phosphorus and cer- 10 tain new compositions of matter resulting therefrom.

This'origlnal application is divided and the respective divisional applications filed as app. Ser. No. 183,794, filed January 7, 1938, and app. Ser. ll No. 183,795, filed January 7, 1938.

One of the objects of this invention is to provide a process for the conversion or ferro-phosphorus, a by-product from the production of elemental phosphorus in a'blast furnace or an 3 electrical reduction furnace, into useful phosphates. Another object of thisinvention is to provide a method for the conversion of ferrophosphorus into phosphates which is more economical than those heretofore used. Other oh- 3. jects of this invention include the production of finely divided iron oxide, the particle size or which isot colloidal dimensions and which may be used as a paint pigment or a rouge, and the production of finely divided basic iron phosphate, 3 the particle size of which is of colloidal dimensions and which may be used as a paint pigment or a fertilizer by the anodic oxidation oi ferrophosphorus in aqueous electrolytes.

Four iron phosphides, FeaP, FezP, FeP, and

35 FePz, which contain 15.6, 21.7, 35.7 and 52.6%

phosphorus respectively, are known. Commercial iron phosphide, commonly called ferro-phosphorus, usually contains 14.5 to 28.5% phosphorus. The commercial material does not,

therefore, contain an amount ot phosphorus corresponding exactly to any one iron phosphide. Perm-phosphorus containing 15.6 to 21.7% phosphorus is usually considered as being a mixture of Pea? and Real, while material containing 4' more than 21.7% is considered a mixture of Fez? and another phosphide richer in phosphorus. Although this description is not limited to term-phosphorus of any one composition, its iron phosphide content is discussed in an illus- 50 trative manner as though it was the iron phosphide, FeP.

I have discovered a process for converting the phosphorus contained in term-phosphorus into phosphates by the anodic oxidation of the ferrophosphorus in anaqueous solution of an electroamended April 30, 1928; 37. 0. G. 757) lyte containing an inorganic compound such as an alkali metal hydroxide, an alkali 'metal -salt of an inorganic acid, an inorganic acid, or a mixture of such a salt with an inorganic acid or an alkali metal hydroxide. The use of an alkali metal hydroxide in the electrolyte'results in the formation of an alkali metal phosphate, the use of an alkali metal salt or inorganic acid inthe electrolyte results in the formation ofa'basic iron phosphate, and the use of inorganic acid in an electrolyte results in the formation of an acid iron phosphate. When mixtures of salts and acids or salts and hydroxides are used as electrolytes the products resemble those obtained with both or the pure ingredients used as electrolytes individually. V

In the several examples which follow, samples of term-phosphorus were made anodes in cells containing aqueous solutions or a base, a 'salt or an acid employing chemically inert cathodes. Electrolysis was carried on in these electrolytes under known conditions of direct current, anode current density, temperature, cell voltage and power input until an appreciable quantity-of term-phosphorus had been consumed, at which time the electrolysis was terminated. The electrodes werethen removed from the electrolytes and the losses in weight of term-phosphorus were determined after cleaning. Any insoluble products present in the electrolytes after electrolysis were filtered out and washed substantially free from the electrolytes with a small amount of water. Wash water used was combined with the filtrate and the total volume measured. The insoluble products were dried, weighed and analyzed fol-the chemical constituents of interest (principally iron, total phosphorus, water soluble phosphate and phosphate insoluble in neutral ammonium citrate solution) The filtrates werev analyzed for their phosphorus pentoxide contents, reported as grams of P205 per liter of solution, and for any other ingredients of interest. The current and energy efliciencies obtained for the oxidation of the phosphorus and ferroe phosphorus were calculated for each experiment irom the ampere hours and kilowatt hours employed, and the amounts of P205 recovered as water soluble phosphate and neutral ammonium citrate soluble phosphate.

One example of the operation oi my process is given for the anodic oxidation of ferrophosphorus in a basic electrolyte. A direct cur rent of approximately 10 amperes was passed for 98.5 hours at an anodic current density of approximately 15 amperes per square foot through sludge, was neve ferro-phosphorus as an anode and graphite as a cathodein 5 liters of potassium hydroxide solution containing 280.5 grams of KOH per liter. At the termination of the electrolysis, the sludge was filtered, washed and dried. A filtrate, 8.05

liters, was found to contain phosphate equivalent to 270 grams of P205 and no R20: (Fe203+AlaOa) The amount of phosphorus converted into soluble phosphate by the use of 1023 ampere hours, as indicated bythe increase in weight of the cathode in a copper coulometer which was in series with the cell during the electrolysis, (0.263 gram of P205 per ampere hour) corresponded to a current eiiiciency of 79.5%. Anaverage cell voltage or 2.0 volts was recorded, corresponding to a total power input of 2.05 kilowatt hours and a power consumption of 3.45 kilowatt hours per pound of total P205 produced. The dried sludge contained 73.3% F820: and 17.7% P305, the latter being water insoluble and insoluble in neutral ammonium citrate solution.

Another example for the operation of my process is given for the anodic oxidation of ferrophosphorus in a saline electrolyte. A direct current of approximately 5 amperes was passed for 137 hours at an anodic current density of approximately 10 amperes per square foot through term-phosphorus as an anode and lead as a cathode in 5 liters of a potassium chloride solution containing 372.75 gramsof KCl per liter. At the termination of the electrolysis, the sludge was filtered, washed and dried. A filtrate, 4.8 liters, was found to contain phosphate equivalent to 2.9 grams P205 per liter. The dried sludge contained total phosphate equivalent to 19.03% P205 and 44.5% FegOa. Although the phosphate contained in the dried sludge was insoluble in water, 70.5% of the total P205 or 13.4% on the basis of the dry ntheless, citrate soluble and, therefore, available from a plant nutrient standpoint. The amounts of phosphorus converted into available phosphate by the use of 582 ampere hours, as indicated by the increase in weight of the cathode in a copper coulometer which was in series with the cell during the electrolysis, (0.190

gram of P205 per ampere hour) corresponded to a current efliciency of 57.5%. An average cell voltage of 1.9 volts was recorded, corresponding to a total power input of 1.105 kilowatt hours and a power consumption of 4.54 kilowatt hours per pound of total available-P205 produced.

Still another example of the operation of my process is given for-the anodic oxidation of ferrophosphorus in an acid electrolyte. A direct current of 5 amperes was passed for 128 hours at an anodic current density of approximately 10 amperes per square foot through a ferro-phosphorus anode and a sheet lead cathode in 5.0

liters of phosphoric acid, containing 188.1 grams of P205 and 0.34 grams of F920: per liter. At'

the termination of the electrolysis the solution resulting, 5.675 liters, was found to contain phosphate equivalent to 177.2 grams of P205 and 27.6 grams of FezO: per liter. The diii'erence between the final and initial P20: contents of the electrolyte of 55 grams PaOt obtained by the use of 582 ampere hours, as indicated by the increase in .weight of the cathode in a copper coulometer in series with the cell during the electrolysis, (0.094

gram of P205 per ampere hour) corresponded to a current efliciency of 28.4% computed on the basis of the oxidation of phosphorus from a negative valence of 3 to a positive valence of 5. An average cell voltage of 1.7 volts was recorded, corresponding to a total power input of 0.99 kilowatt hour and a power consumption of 8.15 kilowatt hours per pound of total P205 produced in solution.

It is evident that there are numerous factors which will influence conditions for the most satisfactory operation of my invention, the actual limits of which cannot be established except by ,a detailed study of each set of raw materials and the intermediate and finished products involved.

Ferro-phosphorus is cast in flat slabs suitable for use as electrodes in electrolytic cells. These slabsmay be cast after the separation of the term-phosphorus from the furnace slag either at the time of tapping the ferro-phosphorus from a phosphate reduction furnace or subsequent thereto. The ferro-phosphorus slabs are made the anodes of a plurality of cells arranged in series electrically and, in order to obtain the maximum anode area, these slabs are mounted centrally between .their respective pairs of cathodes the shunting of individual cells for the replacement of anodes as these are used up or for the removal of electrolyte. introduced discontinuously but it is preferable to permit a continuous admission of theelectrolyte and a continuous withdrawal of some of the partially spent electrolyte containing the phosphate produced continuously.

The disposition of the filtrate and solid products obtained by the anodic oxidation of ferrophosphorus is dependent upon the nature of the originat electrolyte used and the type of product desired.

When an aqueous solution of a. strong-base, such as sodium hydroxide or potassium hydroxide, is employed as the electrolyte, the products obtained from the anodic oxidation of ferro-phosphorus are iron oxide in finely divided form and the tertiary phosphate of the metal of the base originally used as the electrolyte. When the electrolysis has progressed suiiiciently to. produce either a desired concentration of phosphate ions in solution or a sludge of desired density, the electrolyte is pumped to a filter to remove the iron oxide sludge and the tertiary alkali metal phosphate is..recovered by evaporation, cooling, crystallization, and filtration from the mother liquor, which is returned to the electrolytic cells with additional base and water. With proper filtration and washing methods employed; the salt is found to befree from heavy metals-and the iron oxide product will have only traces of the alkali metal phosphate present. The solution containing the alkali metal phosphate may be regenerated after the iron oxide sludge has been removed by the addition of an alkaline-earth metalbase,

is insoluble in alkaline The electrolyte can be.

alkali to make up for the small losses inherent in the process.

When the original electrolyte consists of an aqueous solution of a strong base, the iron oxide sludge-produced is found to be nearly free of phosphorus .at high current densities while at low current densities more unoxidized ferrophosphorus is found in the iron oxide sludge. The phosphorus oxidation current and energy efliciences are considerably higher at low curf I the electrolyte the filtrate obtained after the separation of the basic iron and phosphate is returned to the cells with the addition of the amount-of salt necessary to restore it to its original concentration. The chlorine evolved during electrolysis may be combined with a part of the hydrogen produced at the cathode and may be returned to the system as hydrochloric acid.

When an aqueous solution of a strong inorganic acid -is used, the product obtained is an acid iron phosphate. With a sufliciently high acidity maintained in the electrolyte the oxidation products are all found in solution in the form of iron ions and phosphate ions, otherwise on continuing the electrolysis, the oxidation products will appear as a water insolubleacid iron phosphate. The use of an aqueous solution of phosphoric acid as the electrolyte results in the final production ofwater insoluble iron phosphate containing essentially all the phosphorus added as phosphoric acid and as ferro-phosphorus'. I

The use of electrolytes containing sodium hydroxide, potassium hydroxide, sodium chloride,

potassium chloride, and phosphoric acid have.

been'discussedin considerable detail. This invention is not, however, limited'to the use of these particular electrolytes since solutions of other salts, such as the nitrate, phosphate, or sulfate, and other acids, such as hydrochloric, nitric, and sulfuric, may also be used. Furthermore, the electrolyte may be a solution of a hydroxide and a salt or it may be a solution of a salt and an inorganic acid.

The basic, iron oxide and the basic iron phosphate obtained, using a solution of an hydroxide and a solution of salt, respectively, are believed to be new, particularly in respect to their particle sizewhich is of colloidal dimensions. When these insoluble materials are separated from the electrolyte, washed and dried, they naturally agglomerate on drying. This agglomerate may be readily disintegrated so that the resulting powder may be used for various purposes, such as pigment for use in paint, rouge or rubber, where the colloidal nature of the material imparts distinctly improved characteristics to the resulting products.

Certain terms used throughout the description and claims are understood to have the following meaning: anode current density is the constant current employed divided by the total immersed surface of the anode and is expressed in amperes of phosphorus oxidizedto water soluble and neutral ammonium citrate soluble phosphate divided by the number of grams of phosphorus which would be theoretically oxidized from a negative valence of 3 to a positive valence of 5 .by the use of the number of ampere hours employed, neglecting the amperes hours consumed in oxidizing the iron content of the term-phosphorus; and

energy efiiciency is the amount'of phosphorus which is oxidized to water soluble and neutral ammonium citrate soluble. phosphate per unit of electrical energy. consumed.

It will be seen, therefore, that this invention actually may be carried out by the modification of certain details without departing from its spirit or scope.

I claim:

1. Process of making iron phosphate from ferro-phosphorus, which comprises, making ferrophosphorus an anode in an electrolyte consisting of an aqueous solution of potassium chloride, containing from 10% by weight to a saturated solution; making a conductor, chemically inert as a cathode, a cathode; passing a direct current at an anodedensity of 5 to 50 amperes per square" foot, between the anode and the cathode to produce the insoluble iron phosphate; and separating the insoluble iron phosphate from the electrolyte.

2. Process of making iron phosphate from ferro-phosphorus, which comprises, making ferrophosphorus an anode in an electrolyte consisting of an aqueous solution of an alkali metal salt of a strong mineral acid; making a conductor, chemically inert as a cathode, a cathode; passing a direct current at a current density of 5 to 50 amperes per square foot between the anode and the cathode to produce the insoluble iron phosphate; and separating the insoluble iron phosphate from the electrolyte.

3. Process of making iron phosphate from ferro-phosphorus, which comprises, making ferrophosphorus an anode in an electrolyteconsisting of an aqueous solution of sodium phosphate and sodium hydroxide, containing from 10% by weight to a saturated solution; making a conductor, chemically inert as a cathode, a cathode; passing a direct current, at an anode current density of 5 to 50 amperes per square foot, between the anode and the cathode to produce the insoluble iron phosphate; and separating the insoluble iron phosphate from the electrolyte.

4. Process of making iron phosphate from term-phosphorus, which comprises, making ferro-phosphorus an anode in an electrolyte consisting of an aqueous solution of an alkali metal salt of a strong inorganic acid and an alkali metal hydroxide, containing from 10% by weight to a saturated solution; making a conductor, chemically inert as a cathode, a cathode; passing a direct current, at an anode current density of 5 to 50 amperes per square foot, between the anode and the cathode to produce the insoluble iron phosphate; and separating the insoluble iron phosphate from the electrolyte.

5. Process of making iron phosphate from ferro-phosphorus, which comprises, making ferro-phosphorus an anode in an electrolyte consisting of an aqueous solution of sodium sulfate and sulfuric acid, containing from 10% by weight to a saturated solution; making a conductor, chemically inert as a cathode, a cathode; passing a direct current, at an anode current density of 5 to 50 amperes per square foot, between the anode and the cathode to produce the insoluble iron phosphate; and separating the insoluble iron phosphate from the electrolyte.

6. Process of making iron phosphate from term-phosphorus, which comprises, making term-phosphorus an anode in an electrolyte consisting of an aqueous solution of an alkali metal salt of a strong mineral acid and a strong mineral acid; making a conductor, chemically inert as a cathode, a cathode; passing a direct current, at an anode current density oi! 5 to 50 amperes per square foot, between the anode and the cathode to produce the insoluble iron phosphate; and separating the insoluble iron phosphate from the electrolyte.

7. Process of making iron phosphate from ferro-phosphorus, which comprises, making ferro-phosphorus an anode in an electrolyte consisting of an aqueous solution consisting essentially of an alkali metal salt or a strong mineral acid: making a conductor, chemically inert as a cathode, a cathode; passing a direct current, at an anode current density of not more than 100 amperes per square foot, between the anode and the cathode to produce the phate from the electrolyte.

8. Process of making iron phosphate from term-phosphorus, which comprises, making term-phosphorus an anode in anelectrolyte coniron phosphate.

LEWIS H. D. FRASER.

insoluble iron phosphate; and separating the insoluble iron phos- 

